Method of producing ductile cast iron



Patented Jan. 16, 1951 METHOD OF PRODUCING DUOTILE CAST IRON Lemon, N.Y; aasignor to Union Carbide and Carbon Corporation, a

Charles M. Offenhauer,

corporation of New York No Drawing. Application April 7, 1949, SerialNo. 86,119

4 Claims. I

The invention relates to an improved method for producing ductile castiron and to a new agent for use in such method.

Ductile cast iron may be defined as cast iron which exhibits plasticstrain in tension in an amount equivalent to at least 1% permanentelongation before fracture. Ductile cast iron may be furthercharacterized as cast iron containing an effective amount of graphite ofthe type that appears as spheroids in the as-cast condition. Such iron,due to its ductility and other desirable physical properties,constitutes a valuable material in industry and may be employed in someuses hitherto limited to steels.

It is known that cast iron having a spheroidal or nodular structure maybe prepared from a molten bath of cast iron of specified composition andunder controlled conditions by incorporation into the bath additions ofsuch metals as cerium, titanium, beryllium, lithium and magnesium. Theprior art processes, however, have certain inherent difllculties andlimitations. For example, additions of cerium appear to be effective inthe production of a nodular structure only when added to hypereutecticcast irons of extremely low sulphur content. Additions of elementalmagnesium, however, appear to produce a nodular structure in both hypoand hypereutectic cast irons. Despite this broader range ofapplicability the addition of magnesium to cast iron results in certainserious operational dimculties. When elemental magnesium is added tomolten cast iron there is a violent, almost explosive, reaction. Theviolence resulting from the addition of magnesium not only constitutes amajor hazard to such an operation but, due to expelled cast iron andmagnesium, renders reproducibility of a uniform product by such anaddition uncertain, if not substantially impossible. The addition ofelemental magnesium to cast iron for the production of a nodularstructure is commercially unattractive for another and equally seriousoperational difficulty. Although the introduction of magnesium appearsto be effective in converting both hypo and hypereutectic cast iron tothe desired nodular structure, the effect is of extremely shortduration. This fading effect of the magnesium addition necessitates theimmediate casting of the iron after the magnesium addition andsubseouent innoculation. If casting is delayed even for a matter ofminutes the effect of the addition is lost and a product is obtainedwhich is substantially the same as that which would be obtained if nomagnesium had been added. Since the fading effect of the magnesiumaddition is progressive, there is again the dimculty of reproducibilityof uniform properties in the cast material. A further disadvantage tothe use of magnesium as an addition agent for the purposes mentioned isthe tendency of this element to produce white iron thus rendering thetreated material more sensitive to the formation of chili.

Prior art workers have attempted to ameliorate these difficulties byalloying the magnesium with other metals, for example nickel and copper,prior to its introduction into the molten cast iron. Such alloys whenthe concentration of magnesium is below 30%, preferably even lower, areless violent when introduced into the molten bath. Alloying themagnesium with other metals, however, has not resulted in eliminatingthe objectionable fading effect and accordingly such alloys are stillunattractive in commercial operations. Furthermore, when nickel orcopper is alloyed with magnesium to act as a buffer metal inreducing'the violence of the magnesium addition, the buffer metal isnecessarily incorported in the cast iron. This often may be undesirableparticularly when the cast iron is subsequently used as scrap in thepreparation of other materials. Since neither nickel nor copper isoxidizable in molten iron these metals unavoidably appear ascontaminants in the new material in which the scrap from previous meltsis utilized.

It is a primary object of the present invention to provide an improvedmethod for the production of ductile cast iron which method greatlyextends the interval of time for which the treated cast iron may be heldprior to casting thereby overcoming the objectionable fading effecthereinbefore described.

A further object is to provide a unique agent for use in such methodwhich agent is suitable for the production of ductile cast iron fromboth hypo and hypereutectic cast irons and which is free from thehazards associated with the use of some of the prior art materials.

A still further object is to provide an agent which when employed in themethod of the in vention will decrease the tendency of chill formationinthe treated iron.

The manner in which these and other objects of the invention areattained will be apparent as this specification proceeds.

In the work leading to the present invention it was discovered thatadditions of zirconium to cast iron resulted in the production ofnodular graphite. One limitation on the use of zirconium as an additionagent to cast iron is the difficulty of achieving solution of thezirconium in cast iron and in retaining significant amounts of residualzirconium in the final treated product. It was discovered that elementalmagnesium was an excellent agent for increasing the recovery ofzirconium when the elemental magnesium and zirconium material werecomminuted and incorporated as a mechanical mixture into briquette form.It was found as a most surprising result that molten cast iron treatedwith briquettes containing a mechanical mixture of zirconium andelemental magnesium and innoculated in the customary manner could beheld for appreciably lengthy periods of time with no adverse or fadingeffect and that the last casting poured had substantially the sameexcellent strength, ductility and other physical properties as the firstcasting poured.

According to the invention the new agent comprises a mechanical mixtureof comminuted elemental magnesium and comminuted zirconiumbearingmaterial in a suitably bonded condition. Bonding or briquetting may bedone in any well known manner such as by bonding with a solution ofsodium hydroxide in an amount sufiicient to obtain a strong pellet. Inthe bonded material the ratio of the zirconium content to the elementalmagnesium content should not be more than 3 and preferably about 1, andthe ratio of the elemental magnesium content to the zirconium contentshould not be more than 3 and preferably about 1. The zirconium may beadded to the pellet as a comminuted zirconium-bearin material such ascomminuted zirconium-silicon. or other zirconium-bearing materialprovided the composition is such that the final mechanical mixture iswithin the required composition ranges. Without regard to bindermaterial, the composition of the pelleted mechanical mixture ofcomminuted zirconium-bearing material and comminuted elemental magnesiumshould be within the following ranges:

Broad Prcfvrrod Per cent 1 PM rent Zirconium 10-30 i l53fl H5 5 arms 5455-35 mesh (0.0234 inch screen openings) maximum particle size.

In order to demonstrate the eflectivenes oi the method of the inventionunder actual foundry conditions, a series of test castings were preparedfrom a cupola melted all pig iron charge treated as below.

750 pounds of the cupola melted charge was collected into a bull ladleand treated with 0.50% zirconium and 0.50% magnesium in the form of abriquetted mechanical mixture of comminuted zirconium-silicon alloy andcomminuted elemental magnesium. The nominal analysis of the briquettewas 27% zirconium. 27% magnesium. 35% silicon, the remainder iron. Thebriquettes were placed in paper bags and added to the ladle after thebottom of the ladle was covered with molten cast iron. There wasconsiderable bubbling action in the molten metal upon the addition ofthe briquettes which served to agitate the bath and aid in uniformlydistributing the addition agent throughout the entire molten mass. Theaction was insuflicient to eject any metal from the ladle. The treatediron was Dored from the 750-pound ladle into 40-pound size hand ladlesand innoculated with 0.40% silicon as a silicon-bearing iron alloycontaining about sil con. In order to demonstrate the effect of holdingtime on the properties of the final product, the test castings werepoured over an eight minute interval. There was no decrease inmechanical properties during the time interval investigated, thusdemonstrating the absence of the objectionable fading effect heretoforeencountered in the production of ductile cast iron by prior artprocesses. Data obtained from tensile specimens cut from the keel blockcastings. together with the time at which the cast iron was poured afterthe zirconium-magnesium treatment, are shown in the table below. In thetable casting time after treatment with the briquettes of the inventionis given in minutes and seconds. Y. S. 1000 p. s. i. means yieldstrength in thousand pounds per square inch. T. S. 1000 p. s. i. meanstensile strength in thousand pounds per square inch. Percent elong. in 2in. means percentage elongation in two inches. Percent Red. area meanspercentage reduction in It is understood that the composition may alsosu area at the tensile fracture.

Table P m o n r n fi Be W Y. s. 'r s. Per Cent Per cm gai 1000 1000Elong. Red. m

' p. s. i. p. s. i in 2 in. Area C Si S Mn P Zr Mg Min. Sec.

1 3. 86 3.14 0.033 0. 74 0.19 0. ll 0. 034 4 35 55.5 75. 8 5.0 3.1 2 3.46 3.06 0. 036 0.12 0. 034 5 35 51. 0 72. 9 5. 0 3. 5 3 3. 34 3. 21 0.041 O. 094 0. 050 6 l9 57l0 79. l 4. 5 3. l 4 3. 48 3. 04 0 041 0. 0890. 035 6 50 57. 0 80. 3 4. 5 3. 5 5 3. 33 3. 06 0.033 0.68 0.22 0. 110.030 7 47 56.0 75. 6 6. 0 4. 5

5H Same as Test No. 5 except annealed for 1 hour at 53. 0 78. 75 12. 59. 6

900 C. and iurnace cooled to room temperature.

include such incidental impurities as may be present in thezirconium-bearin material and commercial scrap magnesium metal.

In preparing the materials for the mechanical mixture of the invention,the comminuted zirconium-bearing material should not exceed about 20mesh (0.033 inch screen openings) maximum particle size and thecomminuted elemental magnesium preferably should n xceed. about 30 Itwill be noted from the table that the tensile and yield strengths,percentage elongation and reduction in area of the test specimens areextremely uniform. In fact the specimen taken from test No. 5. pourednearly eight minutes after the briquette treatment, had somewhat betterductility than the specimen taken from the casting of test No. I pouredfully three minutes earlier. The test data clearly establishes thatductile tent of the iron due to holding time aftertreatmentandprlortoeasflng.

The residual zirconium content desired in the ductile cast iron preparedby the method of the invention should be between 0.03% and 0.5%,preferably between. 0.1% and 0.3% and the residual magnesium should bebetween 0.01% and 0.2%. preferably between 0.03% and 0.1%.

Test No. 511 shows the eilect, of heating a test specimen from test No.at 900 C. for one hour. It will be seen that the heat treatment whileslightly lowering the yield and tensile strength had the eflect of morethan doubling the ductility of the specimen as measured by elongationand reduction in area.

In practicing the method of the invention it is desirable that themolten bath of cast iron be within the temperature range of 1400 to 1550C. at the time of the addition of'the agent of the invention.

Innoculation of the treated iron may be made entirely in the bull ormaster ladle after the addition of the agent of the invention or eachcasting ladle of treated molten metal may be separately innoculated.Suiiicient time should be allowed after innoculation for the innoculantto go into solution in the treated iron before pouring. Any electiveinnoculant may be employed such as calcium-silicon, ferrosilicon, othersilicon-bearing alloys, graphite, vanadium and aluminum. Sufllcientinnoculant should be introduced into the treated molten metal tographitize the primary iron carbide.

The presence of phosphorus in amounts normally encountered in cast irondoes not hinder the, ability of the agent and method of the inventlon toproduce spheroidal graphite but does have an adverse eifect upon theductility of the cast material.

The presence of sulphur does not affect the operation of the prom of theinvention but low sulphur is desired as the magnesium in the agent ofthe invention reacis with any excess sulphur necessitating largeradditions of the agent than otherwise would be required. It isrecommended that if the sulphur is higher than desired, the molten castiron be subjected to any well known desulphurization treatment withsodium carbonate. calcium carbide or the like prior to treatment withthe agent of the invention.

The term cast iron as employed in this speciflcation and in the claimsrefers to both hyper and -hypoeutectic iron containing carbon in anamount between about 2% and 4.5%; silicon in an amount between about 1%and 5%; manganese in an amount between about 0.3% and 4%; phosphorusbetween 0.01% and 0.75%, preferably not above 0.25%; and sulphurdesirably as low as practicable but preferably not exceeding 0.2%.Alloying elements such as chromium, molybdenum, and other specialmaterials may be introduced in amounts necessary to give the iron theparticular properties desired without changing its essential character.

What is claimed is:

LInamethodoiproducingductilecastiron,

of cast iron said mechanical mixture of comminuted zirconium-bearingmaterial and comminuted elemental magnesium in an amount sumcient toincorporate in said cast iron a residual zirconium content of between0.03% to 0.5% and a residual magnesium content between 0.01 and 0.2%;thereafter introducing an innoculant to said cast iron in an amountsu'flicient to graphitize the primary iron carbide in said cast iron;holding said molten cast iron until the bath becomes substantiallyquiescent and thereafter casting said molten iron.

2. In a method of producing ductile cast iron, the improvement whichcomprises preparing a bath of molten cast iron within the temperaturerange of about 1400 to 1550 C.; preparing a bonded mechanical mixture ofcomminuted zirconium-bearing material and comminuted elementalmagnesium; introducing into said molten bath of cast iron said bondedmechanical mixture of comminuted zirconium-bearing material andcomminuted elemental magnesium in an amount suflicient to incorporate insaid cast iron a residual zirconium content of between 0.1 and 0.3% anda residual magnesium content between 0.03% and 0.1%; thereafterintroducing an innoculant to said cast iron in an amount sumcient tographitize the primary iron carbide in said 'cast iron; holding saidmolden cast iron until the bath becomes substantially quiescent andthereafter casting said molten iron.

3. An addition agent for the production of ductile cast iron from moltencast iron which agent comprises a mechanical mixture of a comminutedzirconium-bearing material and comminuted elemental magnesium, saidagent containing. in addition to incidental impurities, between 10% and30% elemental magnesium metal, between 10% REFERENCES CITED Thefollowing references are of record in the file of this patent:

Paper No. 875, Institute of British Foundrymen, page 5, paper presentedat the 44th annual meeting of the institute, June 17 to 20, 1947, atNottingham, England.

American Foundryman, February, 1949, pages 30 to 37. Published by theAmerican Foundrymens Society, Chicago, Illinois.

Canadian Mining and Metallurgical Bulletin, July 1949, pages 338 to 348.Published by the Canadian Institute of Mining and Metallurgy, Montreal,Canada.

1. IN A METHOD OF PRODUCING DUCTILE CAST IRON THE IMPROVEMENT WHICHCOMPRISES PREPARING BATH OF MOLTEN CAST IRON WITHIN THE TEMPERATURERANGE OF ABOUT 1400 TO 1550* C.; PREPARING A MECHANICAL MIXTURE OFCOMMINUTED ZIRCONIUMBEARING MATERIAL AND COMMINUTED ELEMENTAL MAGNESIUM;INTRODUCING INTO SAID MOLTEN BATH OF CAST IRON SAID MECHANICAL MIXTUREOF COMMINUTED ZIRCONIUM-BEARING MATERIAL AND COMMINUTED ELEMENTALMAGNESIUM IN AN AMOUNT SUFFICIENT TO INCORPORATE IN SAID CAST IRON ARESIDUAL ZIRCONIUM CONTENT OF BETWEEN 3.03% TO 0.5% AND A RESIDUALMAGNESIUM CONTENT BETWEEN 0.01% AND 0.2%; THEREAFTER INTRODUCING ANINNOCULANT TO SAID CAST IRON IN AN AMOUNT SUFFICIENT TO GRAPHITIZE THEPRIMARY IRON CARBIDE IN SAID CAST IRON; HOLDING SAID MOLTEN CAST IRONUNTIL THE BATH BECOMES SUBSTANTIALLY QUIESCENT AND THEREAFTER CASTINGSAID MOLTEN IRON.